1. Field of the Invention
The present invention relates to an impedance matching device used for a plasma dry process for production of a semiconductor and the like. Particularly, the invention relates to a technique which improves response to a fluctuation of a load impedance. The impedance matching device intervenes, for example, in a transmission path of a high-frequency power between a high-frequency power supply and a load of a plasma chamber or the like. An impedance of the transmission path is matched with an impedance of the load so that reflection of a power from the load is eliminated, and an incident power from the high-frequency power supply is utilized on the load side most efficiently.
2. Description of the Related Art
In a plasma dry process for production of a semiconductor, in recent years, a frequency of a high-frequency power to be used is heightened from an RF band (up to 30 MHz) to a VHF band (30 to 300 MHz) and further to an UHF band (300 MHz to 3 GHz) in order to fine a substrate pattern of a semiconductor element. It is an impedance matching device to supply such a high-frequency power efficiently to a load of a plasma chamber or the like.
The impedance matching device which intervenes in a middle of a transmission path, such as a coaxial tube and a waveguide, of a high-frequency power has a plurality of stubs respectively in positions separated from one another in an axial direction of the tube. A distance of the adjacent stubs is xc2xc of a tube inner wavelength xcexg in the coaxial tube in a frequency of the high-frequency power to be applied.
A plunger type stub has a variable length coaxial tube in which a conductor portion and an outer cylinder portion are provided concentrically, an end short-circuiting electrode which slides in the variable length coaxial tube along the axial direction. In order to make it possible to carry out impedance matching over a wide area of Smith""s chart, a sliding range of the end short-circuiting electrode is generally set to xcexg/4. Moreover, an entire length of the stub is not less than xcexg/2.
The entire length of the stub is supposed to be shortened to about xcexg/4 by eliminating a protruding operation of the plunger and allowing the end short-circuiting electrode to slide in a reciprocating way by means of a wire.
However, in this case, it is necessary to bring each end short-circuiting electrode into close contact with each conductor portion and outer cylinder portion in each stub, and thus frictional resistance of sliding is large. Moreover, since the impedance matching is carried out by adjusting the length of the tube path according to displacement of each end short-circuiting electrode, the entire length of each stub is fairly long, and their accuracy of axial center is low.
Due to the large sliding resistance and the low accuracy of the axial center, a moving speed of the plunger becomes slow, and thus response of the impedance matching to a fluctuation of a load impedance is not good. Further, due to the large frictional resistance of sliding, the end short-circuiting electrode, the outer cylinder portion of each stub, abrasion and deterioration of each conductor portion easily proceed in the long time use, and this decreases the life.
Furthermore, since it is difficult to use the long stub in a laid posture, the impedance matching device is installed in an upright posture, but this occupies a large space of a room in a height-wise direction, and this interferes the installation.
Therefore, a main object of the present invention is to provide an impedance matching device which is capable of being compact and improving response of impedance matching to a fluctuation in a load impedance such as a behavior of a plasma load.
Another object of the present invention is to provide an impedance matching device which is capable of lengthening its life and widening degrees of freedom of installation.
Still another objects, characteristics and advantages of the present invention will become apparent by the following description.
The impedance matching device of the present invention solves the problems mentioned above by taking the following measures.
The impedance matching device according to the present invention includes a plurality of stubs which are provided to a main coaxial tube so to be separated from one another in a premised structure. As for the main coaxial tube, an opening of one end in the tube axial direction is connected to a high-frequency power supply side and an opening of the other end in the tube axial direction is connected to a load side of a plasma chamber and the like. The stubs are serially provided to at least two places of the main coaxial tube separated by a predetermined interval in the tube axial direction in a branch state. In this premised structure, the opening of one end of the main coaxial tube may be connected directly to the high-frequency power supply or connected to the coaxial tube extended from the high-frequency power supply. Moreover, the opening of the other end of the main coaxial tube may be connected directly to the load or connected to the coaxial tube extended from the load. A number of stubs to be provided is preferably three in general, but may be two or not less than four. Directivity of the stubs is normally vertical with respect to the tube axial direction of the main coaxial tube but is not necessarily to be always limited to this. If a stub is extended obliquely, its essentiality does not change. A providing interval of a plurality of stubs is generally and preferably xc2xc of a tube inner wavelength xcexg, but since the interval is not a characteristic itself in the present invention, it is not particularly limited.
According to the present invention, the impedance matching device having the above structure as the premise is characterized by including the following requisites. In other words, each of the plurality of stubs includes a variable capacity capacitor whose one end is to be jointed to an internal conductor of the main coaxial tube, and an electrically conductive capacitor cover to be jointed to an outer conductor of the main coaxial tube so as to be electrically joined to the other end of the variable capacity capacitor as well as to cover a surrounding of the variable capacity capacitor. Further, each stub includes a drive motor which is arranged on an outside of the capacitor cover so as to drive a movable side electrode of the variable capacity capacitor.
The impedance matching device of the present invention executes impedance matching by adjusting an electrostatic capacity of the variable capacity capacitor, unlike a plunger type impedance matching device which executes impedance matching by adjusting a length of a tube path in a stub in accordance with a displacement of an end short-circuiting electrode (short plunger). Unlike the adjustment of the length of the tube path, the adjustment of the electrostatic capacity has large degrees of freedom of space. In the case of the plunger system, the adjustment of the length of the tube path is limited to a relationship of 1:1 in the displacement in the axial direction. In other words, an adjustment amount of the length of the tube path is completely equal with a displacement amount of the end short-circuiting electrode in the axial direction. However, in the case of the adjustment of the electrostatic capacity, for example, cylindrically-shaped movable side electrode and fixed side electrode are inwardly and outwardly fitted to each other so as to form a multiplayer so that the electrostatic capacity can be increased or decreased in a state that the displacement of the movable side electrodes is amplified. Namely, the electrostatic capacity can be changed relatively greatly by comparatively small displacement. Therefore, a moving amount of the movable side electrode for the impedance matching in accordance with a fluctuation of the load impedance may be small. Time required for moving the movable side electrode by a predetermined amount can be shortened in comparison with the plunger system, thereby making it possible to realize high-speed response of the impedance matching.
In the case of the adjustment of the length of the tube path, the end short-circuiting electrode should closely contact with a conductor portion and an outer cylinder portion of the stub so as to slide, but in the case of the adjustment of the electrostatic capacity, such closely contact sliding is not always necessary. Resistance at the time of the displacement of the movable side electrode is reduced greatly, and this is also advantageous to the high-speed response of the impedance matching.
The movable side electrode of the variable capacity capacitor is driven directly by the drive motor arranged on the outside of the capacitor cover. Namely, a reduction mechanism does not intervene, and this is more advantageous to the high-speed response.
In addition, the movable side electrode can be displaced smoothly without resistance, and this is advantageous also to reduce abrasion and lengthen the life.
Since a stroke of the movable side electrode of the variable capacity capacitor can be shortened, a length of the stub itself can be short. It is possible to heighten accuracy of axial center. This high accuracy of the axial center is advantageous to the smooth movement of the movable side electrode and, as a result, advantageous to improve the high-speed response.
Since the stub is short and its accuracy of axial center is high, a posture of the stub is not always limited to an upright posture. For example, the impedance matching device can be installed in a posture that the stubs are laid. Namely, degrees of freedom of the installation becomes high.
An example of the preferable form in the impedance matching device having the above structure is the following structure. Namely, the impedance matching device is designed based on a susceptance of the stubs having the movable capacity capacitance including the electrically conductive capacitor cover which is calculated according to the following formula:                               B          x                =                              ω            ⁢                          xe2x80x83                        ⁢                          C              x                                            1            -                                          ω                2                            ⁢                              C                x                            ⁢                              L                x                                                                        (        1        )            
(where, Bx is the susceptance, Lx is a parasitic inductance, Cx is the electrostatic capacity of the variable capacity capacitor, and xcfx89 is a use angular frequency).
When the variable capacity capacitor reaches a high-frequency range, a level of the parasitic inductance of its internal structural element, particularly bellows and wiring cannot be ignored. Actually, it was found that this disabled the variable capacity capacitor from sufficiently displaying a function as a variable capacity element.
When a complex inductance Z is expressed by a resistance R and a reactance X, the following formula holds:
Z=R+jXxe2x80x83xe2x80x83(2)
When a complex admittance Y is expressed by a conductance G and a susceptance B, the following formula holds:
Y=G+jBxe2x80x83xe2x80x83(3)
where,                     G        =                  R                                    R              2                        +                          X              2                                                          (        4        )                                B        =                  -                      X                                          R                2                            +                              X                2                                                                        (        5        )            
Here, when the resistance R is 0 (R=0), the following formula holds:                     B        =                  -                      1            X                                              (        6        )            
An electrostatic capacity of the variable capacity capacitor according to a displacement amount x from a reference point is represented by Cx. The following formula holds for the reactance X:                     X        =                  -                      1                          ω              ⁢                              xe2x80x83                            ⁢                              C                x                                                                        (        7        )            
Therefore, the following formula holds for the susceptance B:
B=xcfx89Cxxe2x80x83xe2x80x83(8)
The electrostatic capacity Cx is proportional to the displacement amount x from the reference point. When its proportional constant is represented by kc, the following formula holds:
Cx=kcxc2x7xxe2x80x83xe2x80x83(9)
Therefore, when the susceptance B is supposed to be in accordance with the displacement amount x and a symbol Bx is used, the following formula holds:
Bx=xcfx89kcxc2x7xxe2x80x83xe2x80x83(10)
This characteristic curve becomes linear.
The above is adopted to the case where the frequency of the high-frequency power to be used is comparatively low. However, it was found that as the frequency of the high-frequency power became higher, the following problem arose.
In the high-frequency range, the bellows and wiring in the variable capacity capacitor (vacuum capacitor) have parasitic inductance, and its level cannot be ignored. In addition, the capacitor cover which houses the variable capacity capacitor and the wiring have fixed inductance.
Particularly in the frequency of UHF band, the frequency passes a serial resonance point of an LC circuit so that the variable capacity capacitance looses its function and functions as a variable inductor. The variable capacity capacitor in the high-frequency range is considered as a model of a serial resonance circuit having capacitance Cx and inductance Lx of which values change according to the displacement amount x with respect to the reference point. Reactance Xx of the serial resonance circuit becomes as follows:                               X          x                =                              ω            ⁢                          xe2x80x83                        ⁢                          L              x                                -                      1                          ω              ⁢                              xe2x80x83                            ⁢                              C                x                                                                        (        11        )            
When R=0, a susceptance Bx becomes as follows:                               B          x                =                              -                          1                              X                x                                              =                                    ω              ⁢                              xe2x80x83                            ⁢                              C                x                                                    1              -                                                ω                  2                                ⁢                                  L                  x                                ⁢                                  C                  x                                                                                        (        12        )            
Where, xcfx89=2xcfx80f.
Here, due to UHF band, xcfx89Lx becomes very large as represented by the following formula:
1 less than  less than xcfx892LxCxxe2x80x83xe2x80x83(13)
and the susceptance Bx obtains a minus value.
Here, since Bx always has a minus value, the stub can be called as a variable inductance element. Therefore, the following formula holds:                               B          x                =                  -                      1                          ω              ⁢                              xe2x80x83                            ⁢                              L                xe2x80x2                                                                        (        14        )            
Here, when Lxe2x80x2 is inductance when the stub is the variable inductance element, Bx less than 0 because Lxe2x80x2 greater than 0, Lxe2x80x2 becomes as follows according to the formula (13):                                                                         L                xe2x80x2                            =                              -                                                                                                    ω                        2                                            ⁢                                              L                        x                                            ⁢                                              C                        x                                                              -                    1                                                                              ω                      2                                        ⁢                                          C                      x                                                                                                                                              =                                                L                  x                                -                                  1                                      ω                    ⁢                                          xe2x80x83                                        ⁢                                          C                      x                                                                                                                              (        15        )            
An electrostatic capacity Cx of the variable capacity capacitor is subtracted from the parasitic inductance Lx. In the high-frequency range, in order to prevent the stub from resonating, the variable capacity capacitor determines the level of the parasitic inductance Lx of its internal structural element, particularly, the bellows and wiring so that they become as follows:                               L          x                 greater than                   1                      ω            ⁢                          xe2x80x83                        ⁢                          C              x                                                          (        16        )            
In the high-frequency range, it is important to design the variable capacity capacitor and the capacitor cover so that the entire parasitic inductance always has a plus value in a variable range of the variable capacity capacitor.